The present invention relates to local oscillator circuits comprising phase locked loops and, particularly, to such oscillator circuits especially advantageous for use in a tuner for a television receiver.
Television tuners comprising voltage controlled oscillators, phase locked loop synthesizers and the like are becoming increasingly popular due to their stable operation, ease and economy of manufacture and low maintenance. Typical of such a tuner is one which comprises a voltage controlled oscillator and a variable capacitance diode connected to an input of the voltage controlled oscillator. A plurality of potentiometers and push button switches are provided for the respective television channels. The sliders of the potentiometers are set to positions such that when a potentiometer for a particular channel is selected by pressing the respective push button, the voltage appearing at the slider of the potentiometer is applied to the variable capacitance diode through the switch and causes the voltage controlled oscillator to produce a proper local oscillator frequency for the selected channel.
However, such a tuning apparatus must be provided with means for fine tuning, or fine adjustment of the local oscillator frequencies. Such adjustment is necessary, for example, when channel frequencies have deviated from their normal values such as in a cable television system or when two nearby stations broadcast on the same channel with the channel frequencies differing slightly.
The fine tuning function may be considered as providing an offset for the local oscillator frequencies. It is desirable that the range of the offset be the same for all channels and also that the offset range not deviate in frequency. These goals have not been heretofore adaquately attained in the prior art.